Biostratigraphy II

Mass Extinctions

Abrupt simultaneous extinctions of numerous taxa, if real, are powerful biostratigaphic markers and are interesting in their own right. Alas, identifying them isn't straightforward. Identifying a truly abrupt extinction requires continuous deposition and a reasonably dense fossil record.

Biological Caveats:

Even in continuous deposition with a good record, the taxa can be deceptive.

Mindful of these considerations, we see why biostratigraphers employ a variety of zone definitions despite their invocation of conjecture and assumptions: In many circumstances, the ability to bring more data to bear on a problem is simply more important than the avoidance of the fuzziness that follows from inference an conjecture. The biostratigrapher seeks the optimal tradeoff for the specific situation.


The bad news: Constraints on biostratigraphy


The good news:


Biostratigraphic Conventions:

Mindful of these considerations, we see why biostratigraphers employ a variety of zone definitions despite their invocation of conjecture and assumptions: In many circumstances, the ability to bring more data to bear on a problem is simply more important than the avoidance of the fuzziness that follows from inference and conjecture. The biostratigrapher seeks the optimal tradeoff for the specific situation.

Biostratigraphic nomenclature and golden spikes:

Biostratigraphy is the principal determinant of such important things as period boundaries. Boundaries between periods are arbitrarily decided, but usually involve biostratigraphic markers. Some conventions:

E.G. the Silurian-Devonian boundary (right) was decided to be the base of the Monograptus uniformis (a graptolite) zone. Over forty such golden spikes have been established.

How does this help us with the stratigraphy of environments in which fossil preservation is spotty and intermittent?

Biochronology:

When biostratigraphic data is combined with numeric age information we can use biozones as the basis for biochrons - time units! (as opposed to biozones which are rock units)).


Rancho La Brea during the Rancholabrean NALMA from The Page Museum
One famous application: Land Vertebrate Ages. Originally developed during the early and mid-20th century as North American Land Mammal Ages (NALMAs), these were based on the recognition of the regular co-occurrance of distinct mammalian assemblages. NALMAs answered the need for some kind of biochronological tool for North American Cenozoic terrestrial rock units, but were limited by the fact that Cenozoic mammal fossils tended to occur in isolated pockets, often in places with little precise stratigraphic context. (Cave deposits, for example.) As a result, they could not be used as the basis for proper rigorous biozones based on the rocks in which they were found. Instead, they are tied to the time intervals in which the assemblages are inferred to have lived. For regions like the continental deposits of North America, however, NALMAs are much better than nothing, and more frequently mentioned than proper biozones. The NALMAs of the Pleistocene Epoch, for example are good examples and often cited. E.G.:

Over a century of development, competing criteria were used in definitions of ages. Today, biostratigraphers must:

Nevertheless, as a quick-and-dirty, NALMAs/Land Vertebrate Ages are sufficiently useful that they have been expanded globally and pushed confidently into the Late Cretaceous as Land Vertebrate Ages. But note: The use of Land Vertebrate Ages is a tradition among North American paleontologists and stratigraphers originally born of necessity but now, perhaps, just a tradition. Prothero, 1995 reviews the application of new data from improved radiometric dating and magnetostratigraphy, and concludes that the time is ripe for incorporation of NALMAs into proper biozone-based biostratigraphy.

All rest on the assumption that biostratigraphic units are good proxies for time. As indicated above, this seems to be a good first-order approximation.

Final thoughts

Quantitative Biostratigraphy

Besides hopefully constraining their age and sequence, does biostratigraphy add to our kowledge of the deposition of sediments? Actually, yes.

Graphic correlation: method for stratigraphic correlation based on statistical correlation of first and last appearances, but not biozone terminology. Facilitates comparison of locality sections containing local FADs and LADs of the same taxa. Used to:

Composite standards: The examples above correlate teilzones from pairs of localities. On a larger scale, the data used to achieve this can be combined into substantial composite standard databases that: Comparison of information from a particular locality with the composite standard facilitates robust identification of range extensions, hiatuses, and outliers.

Moral: No method of biostratigraphy lacks significant biases and limitations, however biostratigraphy compensates by enabling stratigraphers to bring an overwhelming quantity of data to bear, making it cumulatively very powerful.